You could make your own "feed" pretty easily. I do with my horses and they are doing very well! Mine are also on free choice grass hay and some grazing. They only have a plain salt block as the mineral blocks don't really provide much in the way of minerals and they can have heavy metals in them, which are not good.
I make my own feed using alfalfa pellets, chopped/bagged alfalfa hay, plain whole oats, flax meal, and a vit/min supplement.
The chopped hay helps slow the horse's down, so they don't wolf down the pellets. It also provides more volume without more weight, so the horse thinks she's getting more food than she really is
Grand Vite is one of the best off the shelf mixes. Horsetech.com also has an excellent one called High Point. They also have a very high quality flax meal. The other vit/min I would recommend would be Uckele's Equi-Base Grass.
To start with, I would get a 3 qt feed scopp and start her on 1/2 scoop of chopped hay (about 12 oz), 1/2 scoop of alfalfa pellets (about 1 lb 10 oz), and 1/2 cup of flax (2 oz by weight), with your vit/min supplement, once a day. Keep her on that for 30 days. If she holds her weight nicely, stick with that. If she gets too fat, cut back on the pellets. If she starts to lose weight, increase to 1 scoop of pellets and 1 cup of flax.
The oats I use just as variety. I feed 1 cup to 1/2 scoop. It's not necessary, but can be fun to add. 1/2 scoop usually is about 1 lb of plain whole oats.
My horses have all turned in to easy keepers since going "all natural" with their diets. Even my hardest worker gets just 1 scoop of hay, 1/2 scoop of pellets, 1/2 scoop of oats, and 1/2 cup of flax. That's 1/3 rd the amount of food that she got at her previous owner's house and she is in heavier work with me! She was getting 4- 3qt scoops of strategy and 2- scoops of sweet feed a day, split in to two feedings.